Fast response membrane generator using heat accumulation

ABSTRACT

A membrane gas generator, which contains: a) a feed stream compressor having a feed stream input, b) a heat source downstream of the feed stream compressor, and in fluid connection therewith, c) a heat accumulator downstream of the heat source, and in fluid connection therewith; and d) at least one membrane with a permeate side and a non-permeate side downstream of the heat accumulator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fast response membrane generatorusing heat accumulation.

2. Description of the Background

Membrane processes are presently used in a wide variety of gasseparators. In such processes, the feed stream is brought into contactwith the surface of the membrane with the more readily permeablecomponent being recovered at low pressure, whereas the less readilypermeable component is collected as a non-permeate stream at a pressureclose to the feed pressure.

Membrane systems are increasingly employed, for example, for on-siteproduction of nitrogen or other gases. In all such cases, the membranegenerator is used in combination with a liquid nitrogen back-up tankthat is used at start-up until the production reaches the requiredpurity as requested by the customer, during peak demands or generatorshutdown periods. Unfortunately, the efficiency of such membrane systemsis reduced by variable customer demand over time. In particular, whenthe required flow rate decreases, the system is utilized only part-timeso that stops and restarts are frequent. Furthermore, long-termshutdowns frequently occur due to required regular maintenance. Thus,liquid nitrogen must be vaporized at each restart to feed the customerline before the generator has reached the required level of purity. Thisvaporized liquid nitrogen is, in any case, more expensive than thenitrogen produced by the generator. Thus, there is a great need toreduce the consumption thereof to a minimum. It is also especiallydesired to reduce the start-up time to a minimum.

One of the limiting factors for reaching the required purity is the timerequired for the membranes to reach operating temperature when thistemperature is above ambient temperature. In cases, where the membraneoperating temperature is above ambient temperature, it is, therefore,necessary to be able to introduce a large quantity of heat into thesystem in a very short period of time, such as a few seconds.Unfortunately, at present, several minutes are required to reachoperating temperature. Most of the time required to bring the system tooperating temperature is due to the inertia of the heater, heating ofthe pipes and membranes that are cold.

At present, in order to reduce start-up time, a system has been proposedwhich uses an electrical gas heater on the feed pipe. Although verypowerful heaters may be used, this amounts to over-design at excessivecost when compared to the power required merely to keep the system atoperating temperature.

Another solution proposed is to maintain the entire system at therequired temperature by maintaining a high ambient temperature. However,this turns out to be quite energetically expensive, particularly forlong-term shutdown periods. Even if only the sensitive part, i.e., themembranes, are enclosed in a heated insulated box as in U.S. Pat. No.4,787,919, the cost may still be high for long-term shutdown periods.Moreover, this does not improve the time necessary to heat the pipes andother equipment that is installed between the heater and the membranes.Further, if the air feed to the system is cold, the membranes will notwork at optimum even if they are already at the operating temperature.Additionally, since the mass of the membrane is small compared to thefeed flow rate, the system operates with the inefficiency of a coldsystem if the feed air is cold.

Thus, a need exists for a membrane generator system that can berestarted in a short amount of time, reaching operating temperaturequite rapidly without any additional cost in energy or in investment.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a heataccumulator for a membrane gas generator that stores part of the heatprovided by a heater during operation and which can restore heatimmediately to feed air at start-up when large power is required andwhen the heater is started.

It is also an object of the present invention to provide a heataccumulator as part of a heat buffer for a membrane gas generator.

The above objects and others which will become more apparent in view ofthe following disclosure are provided by a membrane gas generator, whichentails a) feed stream compressing means; b) heat source means in fluidconnection with and downstream of the feed stream compressing means; c)heat accumulator means, in fluid connection with and downstream of theheat source means; and d) at least one membrane having permeate side andnon-permeate side downstream of the heat accumulator means and being influid connection therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates heat accumulator means used in a membrane nitrogengenerator between heat source means and the membrane in accordance withthe present invention.

FIG. 2 illustrates the use of an activated carbon tower as heataccumulator means.

FIG. 3 illustrates different membrane feed gas temperature profiles as afunction of apparatus used, where the temperature profile using theprior art methodology is shown.

FIG. 4 represents a temperature profile using the two-stage control ofthe prior art.

FIG. 5 represents a temperature profile obtained using the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a membrane gas generatorapparatus is provided which is capable of being restarted in a veryshort amount of time, and reaching operating temperature quite rapidlywithout any additional cost in energy or in investment.

The present invention also provides a heat accumulator means which iscapable of storing a portion of the heat provided by heat source meansduring normal operation of the generator. Hence, the heat source meanscan be used to add a quantity of energy to the heat required to maintainthe feed gas at a required temperature or an external heat source meansmay be used to compensate for the thermal losses in the accumulator.When the unit is restarted, the stored energy may be restored as thesystem will be cold after shutdown.

Further, in accordance with the present invention, the heat stored mayprevent the membranes from overheating, acting as a heat buffer. This isparticularly important when the operating temperature is close to themaximum temperature tolerated by the membranes.

While it is generally of practical benefit to operate a membrane at arelatively high temperature, there are practical considerationsregarding the operating temperature which is used. These considerationsmay be due to the membrane, itself, the mechanical properties of whichgenerally decrease when temperature increases, or they may be due toother construction materials used in the membrane module, e.g.,tube-sheet material, baffle material or shell material.

For example, a nitrogen separation membrane may be constructed ofpolyimide with tube sheets made of epoxy. In this case, the epoxymaterial used for tube-sheet manufacture is the limiting factor foroperation at elevated temperatures. In particular, such material loses asubstantial portion of its mechanical properties at temperatures above95° C. Generally, therefore, the maximum long term operating temperaturefor such a membrane is no more than about 80° C., to provide an adequatemargin of safety.

Generally, in accordance with the present invention, a membrane gasgenerator is provided which contains feed stream compressing meanshaving feed stream input means; heat source means downstream of the feedstream compressing means, and which is in fluid connection therewith;heat accumulator means downstream of the heat source means, and which isin fluid connection therewith; and at least one membrane with a permeateside and a non-permeate side downstream of the heat accumulator means.

In more detail, the heat accumulator means of the present invention maybe any type of solid material having a sufficiently high specific heat.For example, brick or refractory materials may be used. The heataccumulator substance may also be activated carbon which can be used ina carbon tower, for example, in a process used to remove oil from feedair. See FIG. 2.

Generally, the activated carbon tower is used for two functions. Thefirst function is for oil vapor removal through adsorption in activatedcarbon pores. The second function is to act as a heat accumulator, i.e.to be able to store heat energy through one of the means describedabove, and to deliver it to feed gas when necessary.

Any conventional solid material may be used having a specific heat whichis sufficiently high to render the material susceptible to use in theheat accumulator means. Materials meeting such a level of specific heatare well known to those skilled in the art.

Generally, any material may be used which has a sufficiently highspecific heat, low cost and good chemical stability. As examples,pebbles, brick or steel balls may be mentioned. However, any materialmeeting the general conditions described above may be used. For example,reference may be made by the artisan to the Chemical Engineer's Handbook(Perry & Chilton, 5th Edition).

Additionally, a heat accumulator material may be used which retainsenergy due to its inherent thermal inertia or it may require anadditional heating supply to compensate for the losses during long-term,shutdown periods. The heat accumulator material may be inserted in aninsulated shell alone or in an insulated shell that also contains themembranes.

As used herein, the term "inherent thermal inertial" refers to the factthat in some instances it may be unnecessary to add heat to theaccumulator if during the period of observation heat loss to the outsideof the accumulator is negligible compared to the heat stored initiallyin the accumulator.

The heat source means may be any conventional heat source, such as forexample, an electrical heater or a heat exchanger using either aninternal or an external energy source. Such heat source means are wellknown to those skilled in the art.

As used herein, the term "internal/external energy source" means thatenergy used is recovered within the generator. For example, it may be apartial recovery of the air compression energy. In practice, this can beperformed using an air-oil heat exchanger on the oil circuit of alubricated screw compressor. Further, the external energy source may bean outside electrical heater, or a steam heater or a hydrocarboncombuster, for example.

Those skilled in the art will appreciate that various changes andmodifications can be made in the details of the invention withoutdeparting from the scope of the invention as set forth in the appendedclaims. Thus, the permeable membranes employed in the practice of theinvention will commonly be employed in membrane assemblies typicallypositioned within enclosures to form a membrane module comprising theprincipal element of a membrane system. As understood with reference tothe invention, a membrane system comprises a membrane module or a numberof such modules, arranged for either parallel or series operation. Themembrane modules can be constructed in convenient hollow fiber form, orin spiral wound, pleated flat sheet membrane assemblies, or in any otherdesired configuration. Membrane modules are contracted to have a feedair surface side and an opposite permeate gas exit side. For hollowfiber membranes, the feed air can be added either to the bore side or tothe other surface side of the hollow fibers.

It will also be appreciated that the membrane material employed for theair separation membrane and for the hydrogen purification membrane canbe any suitable material capable of selectively permeating a morereadily permeable component of the feed gas, i.e., air or impurehydrogen. Cellulose derivatives, such as cellulose acetate, celluloseacetate butyrate and the like; polyamides and polyimides, including arylpolyamides and aryl polyimides; polysulfones; polystyrenes and the like,are representative of such materials. However, in accordance with thepresent invention, polyimides are preferred.

As indicated above, the permeable membranes comprising the membranesystem positioned within the insulated enclosure of the invention may bein any desirable form, with hollow fiber membranes being generallypreferred. It will be appreciated that the membrane material employed inany particular gas separation application can be any suitable materialcapable of selectively permeating a more readily permeable component ofa gas or fluid mixture containing a less readily permeable component.Cellulose derivatives, such as cellulose acetate, cellulose acetatebutyrate, and the like; polyamides and polyimides, including arylpolyamides and aryl polyimides; polysulfones; polystyrenes and the like,are representative examples of such materials. It will be understood inthe art that numerous other permeable membrane materials are known inthe art and suitable for use in a wide variety of separation operations.As noted, the membranes, as employed in the practice of the invention,may be in composite membrane form, in asymmetric form or in any suchform that is useful and effective for the particular gas separationbeing carried out using the system and process of the invention.

The hollow fiber membranes are generally formed from a polymericmaterial which is capable of separating one or more fluids from one ormore other fluids in a fluid mixture. The polymeric materials which maybe used to prepare the hollow fiber membranes preferably includeolefinic polymers, such as poly-4-methylpentene, polyethylene, andpolypropylene; polytetrafluoroethylene; cellulosic esters, celluloseethers and regenerated cellulose; polyamides; polyetherketones andpolyetheretherketones; polyestercarbonates and polycarbonates, includingring substituted versions of bisphenol based polyestercarbonates andpolycarbonates, polystyrenes; polysulfones; polyimides;polyethersulfones; and the like. The hollow fiber membranes may behomogeneous, symmetric (isotropic), asymmetric (anisotropic), orcomposite membranes. The membranes may have a dense discriminatingregion which separates one or more fluids from one or more other fluidsbased on differences in solubility and diffusivity of the fluids in thedense region of the membrane. Alternatively, the membranes may bemicroporous and separate one or more fluids from one or more otherfluids based on relative volatilities of the fluids.

Hollow fiber membranes with dense regions are preferred for gasseparations. Asymmetric hollow fiber membranes may have thediscriminating region either on the outside of the hollow fiber, at theinside (lumen surface) of the hollow fiber, or located somewhereinternal to both outside and inside hollow fiber membrane surfaces. Inthe embodiment wherein the discriminating region of the hollow fibermembrane is internal to both hollow fiber membrane surfaces, the inside(lumen) surface and the outside surface of the hollow fiber membrane areporous, yet the membrane demonstrates the ability to separate gases. Inthe embodiment wherein gases are separated, the preferred polymericmaterials for membranes include polyestercarbonates, polysulfones,polyethersulfones, polyimides, and polycarbonates. More preferredpolymeric materials for gas separation membranes include polycarbonatesand polyestercarbonates. Preferred polycarbonate and polyestercarbonatemembranes for gas separation include those described in U.S. Pat. Nos.4,874,401, 4,851,014, 4,840,646, and 4,818,254; the relevant portions ofeach patent incorporated herein by reference for all legal purposeswhich may be served thereby. In one preferred embodiment, such membranesare prepared by the process described in U.S. Pat. No. 4,772,392, therelevant portions incorporated herein by reference for all legalpurposes which may be served thereby.

Having described the present invention, reference will now be made tocertain examples which are provided solely for purposes of illustrationand are not intended to be limitative.

EXAMPLE 1: Accumulator Design

If, for example, the generator feed flow rate is 100 Nm³ /h and thetemperature of which must be increased from 20° C. (ambient temperature)to 40° C. (operating temperature), the electrical heater is designed asfollows:

    POWER=(F.sub.air * ΔT C.sub.pair)+thermal loss

Where:

F_(air) =air flow rate=100 Nm³ /h

ΔT=temperature variation to bring to the gas=40-20 =20° C.

C_(pair) =air specific heat=0.25 cal/g.° C.

Therefore: POWER>0.75 kV

However, the time required to heat all the pipes and system beforereaching the equilibrium is around 10 minutes. Most of the poweravailable from the electrical heater is used to heat the system and notto heat the feed air. In that case, the extra energy required by thesystem to complement the feed air heating during the 10 minutes lap is:

E=(F_(air) * ΔT*C_(pair))*Time

E=450 kJ

This energy may be provided by the present heat storage system. It mustbe designed so that:

    E=M.sub.s *C.sub.ps *ΔT.sub.s

Where:

M_(s) =accumulator mass

C_(ps) =accumulator specific heat

ΔT_(s) =accumulator temperature variation

It can reasonably be assumed that the heat transfer between theaccumulator and the feed air is ideal, i.e., that the air temperatureand the accumulator temperature are equal.

Further, it is acceptable to feed the membranes with a gas temperatureslightly lower than 40° C.; for example, 38° C. would still beacceptable.

Therefore:

ΔT_(s) =40-38=2° C.

E=450 kJ=(M₈ *C_(ps))*2

This may be achieved with:

80 liters of magnesite brick (C_(ps) =0.93 J/g.° C., density=3000 kg/m³)or with 110 liters of stones (C_(ps) =0.84 J/g.° C., density=2500 kg/m³)or with 400 liters of activated carbon in the case of scheme 2 (C_(ps)=1.05 J/g.° C., density=500 kg/m³).

The required volumes are very compatible with the size of practicalunits. The accumulator material may also be a material that has a phasetransition at the operating temperature. Such a material would thenrestore the energy stored in the high temperature phase without anychange of temperature (ΔT,=0) which is optimized.

The stabilization time for a membrane generator would then decrease from10 minutes to less than 1 minute, reducing the liquid nitrogen vaporizedto a minimum. In the absence of peak demands supplied by liquidnitrogen, the consumption will be reduced by a factor of 10. If only 50%of the liquid nitrogen is due to start-up time supply, the global liquidconsumption will drop to 55% of its previous value.

EXAMPLE 2 Protection from Overheating

In accordance with the present invention, it is also possible to adjustthe heating control parameter in order to decrease the stabilizationtime from 10 minutes to 5 minutes for example. However, in such a case,there is a risk of overheating the membranes. This is represented inFIG. 3. This is especially undesirable when the operating temperature isclose to the maximum tolerated temperature.

Using a conventional methodology, a two-stage control system is used toprevent the membranes from overheating, where the temperature is broughtto the operating temperature minus 5° C. if we expect the overheating tobe around 5° C. When the first plateau is reached, then the controlsystem may switch to a second step to reach slowly the operatingtemperature. In any case, that solution does protect the membranes fromoverheating, but the stabilization time may then be very long.

By contrast, in accordance with the present invention, the accumulatorcan also be used as a heat buffer, so that the temperature seen by themembranes is almost constant. This is represented in FIG. 5. Theconventional double-staged control system is, thus, unnecessary and thetime at start up is reduced while a good protection is provided againstoverheating.

Finally, in order to more fully explain the advantages of the presentinvention reference will now be made to each of the Figures in detail.

FIG. 1 illustrates the use of and positioning of the heat accumulatormeans in accordance with the present invention.

FIG. 2 illustrates an activated carbon tower used as a heat accumulatoror storage means in accordance with the present invention.

FIG. 3 illustrates different membrane feed gas temperature profiles,where a temperature profile is shown using the prior art methodology.

FIG. 4 represents a temperature profile using the two- o stage controlof the prior art.

FIG. 5 represents a temperature profile obtained using the presentinvention.

Generally, in accordance with the present invention any membrane ormembranes may be used which are conventionally used for gas separation.For example, mention may be made of polyimides, polycarbonates,poly(phenylene oxide), nylon-6,6, polystyrene and cellulose acetate. Infact, any one or combination of these membranes may be used where morethan one membrane is used. However, in accordance with the presentinvention, it is particularly advantageous to use one or more polyimidemembranes at relatively high temperatures.

Further, in accordance with the present invention, it is advantageous toreheat the feed air or the feed air line after compression, before themembrane inlet. It is preferred, however, that the feed air be reheatedat a location which is in close proximity or near to the membranemodule(s) inlet(s).

By reheating the feed air at a location which is in close proximity toor near the membrane module inlet(s), a better control of airtemperature is maintained, regardless of outside conditions. Further,with such an arrangement, heat consumption may be minimized.

Generally, it is desirable to reduce the conduit distance between theheater and the module inlet(s). More particularly, it is preferable touse as short of a length of pipe as possible between the heater and themodule inlet(s). In general, the distance is close enough to themembrane inlet(s) for outside temperature variations to have anegligible effect on feed air temperature at the module inlet(s). By"negligible effect" is generally meant that the temperature fluctuationsare under +1° C.

In accordance with the present invention, individual components used mayeach be conventional, such as compressing means, filtration means, heatsource and membrane means. However, the incorporation of the presentheat accumulator means into the membrane gas generator of the presentinvention affords an integrated system having surprisingly advantageousfeatures. Further, the ability of the present generator to rapidlyrestart in a highly efficient manner, itself, affords a surprisinglyeffective method of use therefor.

Furthermore, in accordance with the present invention a feed air heateris generally employed. However, one or more additional heating means mayalso be used.

For example, it is advantageous to employ heating means, such as aheater, which is integrated into the heat accumulator, itself,independent of the feed air heater and in addition to it. Generally,such additional heating means may be controlled by a temperature sensorlocated in the heat storage material, and heat may be added whenever theheat storage material temperature falls under a preset level.

Having described the present invention, it will now be apparent to oneof ordinary skill in the art that many changes and modifications may bemade without departing from the spirit and the scope of the presentinvention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method of rapidly restarting a membranenitrogen generator which is operated non-continuously, thereby reachingoperating temperature in a reduced amount of time, said generatorcomprising feed stream compressing means having feed stream input means;heat source means downstream of said feed stream compressing means, andin fluid connection therewith; heat accumulator means downstream of saidheat source means, and in fluid connection therewith; and at least onemembrane with a permeate side and a non-permeate side downstream of saidheat accumulator means; said method, comprising:a) storing a portion ofheat provided by said heat source means during operation of thegenerator in said heat accumulator, and b) restoring heat to thegenerator from said heat accumulator means after said generator has beendown and upon restarting said generator.
 2. The method of claim 1,wherein said heat is restored to a compressed gas stream delivered bythe compressing means.
 3. The method of claim 1, wherein said heat isrestored to said at least one membrane.
 4. The method of claim 3,wherein said at least one membrane is operated at a temperature which isnear maximum temperature tolerated by the at least one membrane.
 5. Themethod of claim 1, wherein said heat source means is one using either aninternal or external energy source.
 6. The method of claim 5, whereinsaid internal energy source obtains energy from a partial recovery ofair compression energy.
 7. The method of claim 5, wherein said externalenergy source is an electrical heater, steam heater or hydrocarboncombuster.
 8. The method of claim 1, wherein said heat accumulator meanscomprises a material having a specific heat which is sufficiently highto enable said material to accumulate and retain heat.
 9. The method ofclaim 1, wherein said heat accumulator means comprises a material havingan inherent thermal inertia sufficient to accumulate and retain heat.10. The method of claim 1, wherein said heat accumulator means iscontained, by itself, in an insulated shell or is contained in aninsulated shell also containing said at least one membrane.
 11. Themethod of claim 1, wherein said heat accumulator means satisfies thefollowing equation:

    E=M.sub.s *C.sub.ps *ΔT.sub.s

wherein E is the required energy input, M is the accumulator mass, Cpsis the accumulator specific heat and ΔT_(s) is the temperature variationof the heat accumulator.
 12. The method of claim 1, wherein said atleast one membrane is made of a material selected from the groupconsisting of polyimide, polycarbonate, poly(phenylene oxide), nylon-6,6polystyrene and cellulose acetate.
 13. The method of claim 1, whereinsaid at least one membrane is a polyimide membrane.
 14. The method ofclaim 1, which further comprises heating means proximate to an inlet forsaid at least one membrane, which is in a module, for reheating feedair.
 15. The method of claim 14, wherein said heating means ispositioned relative to said membrane module inlet(s) such that outsidetemperature variations cause a temperature fluctuation of less than +1°C. on feed air temperature.
 16. The method of claim 1, which furthercomprises one or more additional heating means, which heating means isintegrated into the heat accumulator, and is independent of the heatsource means downstream of said feed stream compressing means.
 17. Themethod of claim 16, wherein said heating means integrated into said heataccumulator is controlled by a temperature sensor located in the heataccumulator.